| Abstract
| - Reichardt's ET(30) as well as Kamlet−Taft's α (hydrogen-bond-donor acidity) and π* (dipolarity/polarizability)values of various solid acids, e.g., silicas, aluminas, alumosilicates, titanium dioxides, and alumina-functionalized silica particles, are presented. The ET(30) values of the solid acids were directly measured byUV/vis spectroscopy using 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate (1a) and an eicosafluorine-substituted derivative of 1a (1b) as surface polarity indicators. Kamlet−Taft's α and π* parameters for thevarious solid acids are analyzed by means of Fe(phen)2(CN)2 [cis-dicyano-bis(1,10-phenanthroline)iron(II)](2) and Michler's ketone [4,4‘-bis(N,N-dimethylamino)benzophenone] (3) as solvatochromic surface polarityindicators. The chemical interpretation of the α and π* parameters and the nature of the surface sites whichthey reflect are discussed. The correspondence of the UV/vis spectroscopic results to those of the IR-sensitiveprobe benzophenone and the fluorescence probe pyrene (literature data) is also discussed. It is evident thatthe solid surface environments observed by the various indicators are moderately strong dipolar/polarizable(π* = 0.38 to 1.04) and are fairly strong hydrogen-bond donors (α = 1.00 to 1.99). Theoretical ET(30)values of solid acids are calculated by applying linear solvation energy (LSE) relationships using theindependently measured α and π* values of the solid acids according to ET(30) = [ET(30)]o + aα + sπ*.The respective dependence of the α and π* terms, expressed by the coefficients a and s, upon the measuredET(30) value for solid acids is discussed in comparison to related multiple LSE correlations known for well-behaved regular solvents and functionalized silicas. The results show in general that values of polarityparameters of strong HBD and dipolar surfaces are strongly influenced by the experimental conditions applied.
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